1. Field of the Invention
This invention relates to an improvement of a device for supplying a replenishing toner to a developing unit provided in a dry type electrophotographic copying machine.
2. Description of the Prior Art
In electrophotography an electrostatic latent image is generally formed on the surface of a photoconductive insulated layer by known methods of electric charging and image wise exposure. The latent image is developed by magnetic brush. It is considerably troublesome generally in the construction of a developing means, which includes a carrier (iron, etc) brush using a magnet, to replenish the developing means with a developer, which is a mixture of carrier and toner. As is generally known, the toner content in the developer is decreased as the toner is consumed during copier operation. The construction of the conventional electrophotographic copying machine of known type has an automatic toner supplier to provide the developer in a developing unit with an amount of toner corresponding to the amount of toner consumed in the developing unit during copying operation. Such copying machines are ideal in operation, but the mechanisms which operate to automatically supply the toner by detecting the concentration of the toner in the developer by a detector results in a very complicated machine. For example, U.S. Pat. No. 3,616,969 discloses a copier in which the replenishing toner is constantly supplied. U.S. Pat. No. 3,572,922, for example, discloses the supplying of the replenishing toner in accordance with the charging amount. Furthermore, a device has not yet been developed for accurately measuring the concentration of the toner in the developer to control the toner supply amount thereof to prevent poor copying operation.
The toner supply devices generally used at present operate to supply a predetermined amount of toner corresponding to the amount substantially consumed in every copying operation of predetermined number of copy sheets. However, the types of originals to be copied, such as letters, photographs in a catalog, patterns, etc., cause the toner consumption to be different depending upon the type of the original documents. More particularly, when a large number of original documents having much black ground is copied, the amount of toner consumed is excessive, whereas when a number of original documents having much white ground is copied, the amount of toner consumed is much less, and accordingly, consistently good copies cannot be obtained. That is, it is not possible to maintain the toner content constant merely by replenishing the toner at a constant rate, irrespective of the varying optical densities of the originals. A device is known for adjustably supplying a toner of the amount in response to the toner consumption, but such device still has disadvantages. The toner concentration of the copied picture becomes irregular since the toner supply adjusting means used at present do not respond to the various darkness of the picture on a variety of original documents.